1. Technical Field of the Invention
This invention relates to telecommunication systems and, more particularly, to a method of detecting and inhibiting mobile station handoff oscillations between cells in a cellular radio telecommunications network.
2. Description of Related Art
Mobile stations (MSs) will sometimes be handed off back and forth in a fairly rapid manner between two or more cells. This phenomenon is known as handoff oscillations, and has many adverse effects in the cellular network. First, temporary muting is experienced by the subscriber during each handoff, and this adversely affects voice quality when it is repeatedly experienced in a short period of time. Second, the probability of dropping the call is cumulatively increased since the probability of dropping a call is increased during each handoff. Third, the repeated handoffs add to the processor load in the network.
One of the main causes of handoff oscillations is the manner in which MSs obtain signal strength measurements from the serving cell and candidate neighboring cells. Erroneous signal strength measurements from neighbor cells can be caused by a design defect in certain MSs or by poor cell planning causing co-channel interference or poor propagation in certain areas. Therefore, MSs often do not report the correct signal strengths, making the signal strength from the neighbor cell appear better than it actually is, and better than the serving cell. The handoff is then performed based on the erroneous measurements. This places the MS in a new serving cell where the measurements then show that the signal strength is better in the old serving cell. Therefore, another handoff is performed back to the old serving cell.
Although there are no known prior art teachings of a solution to the aforementioned deficiency and shortcoming such as that disclosed herein, U.S. Pat. No. 5,623,535 to Leung et al. (Leung), U.S. Pat. No. 5,530,910 to Taketsugu (Taketsugu), and U.K. Patent Application GB 2 301 733A to Gilliland (Gilliland) discuss subject matter that bears some relation to matters discussed herein. Leung discloses a method of operating a layered cellular network having macrocells which overlay a plurality of microcells. Leung compiles mobility statistics for each MS and passes the statistics to the base stations in the networks. The mobility statistics are then utilized to make decisions regarding whether to assign each MS to a microcell or a macrocell. However, Leung does not teach or suggest a method of detecting and inhibiting handoff oscillations in a cellular telecommunications network.
Taketsugu discloses a method of controlling handoffs in a cellular telecommunications network having cells of differing sizes. Taketsugu measures the number of handoffs experienced by each MS, and the measurement is then utilized to determine the size of cell to which the MS should be handed off. Taketsugu, however, does not teach or suggest a method of detecting and inhibiting handoff oscillations in a cellular telecommunications network.
Gilliland discloses a method of controlling handoffs in a cellular telecommunications network having macrocells and microcells. Gilliland determines the speed of each MS by counting the number of handoffs which have occurred in a preceding period, or by counting the number of measurement reports in the preceding period. The MS's speed is then utilized to determine whether to hand off the MS to a macrocell or a microcell. However, Gilliland does not teach or suggest a method of detecting and inhibiting handoff oscillations in a cellular telecommunications network.
Other solutions to the handoff oscillation problem have been proposed, but each known proposed solution utilizes a hysteresis value to control the rate of oscillating handoffs. In order for a handoff to be performed, the signal strength from a candidate neighboring cell must exceed the signal strength of the serving cell by the hysteresis amount. However, a disadvantage to this approach is that every MS in the cell is affected when the hysteresis value is determined and set. The hysteresis value necessarily reduces the network's handoff performance. For MSs experiencing severe handoff oscillation problems, this trade-off is acceptable. However, all MSs are not uniformly affected by oscillating handoffs. For those MSs that are not experiencing oscillating handoffs, the hysteresis solution degrades their performance without any counter-balancing benefit. In addition, other factors such as interference may require that the value of the hysteresis be set higher, thereby further degrading handoff performance for all MSs in the cell.
Review of each of the foregoing references reveals no disclosure or suggestion of a method such as that described and claimed herein. In order to overcome the disadvantage of existing solutions, it would be advantageous to have a method of detecting and inhibiting handoff oscillations between cells in a cellular radio telecommunications network. Such a method would operate on a per-mobile station basis rather than utilizing hysteresis on a cell-wide basis. The present invention provides such a method.